Extracting core samples from subterranean formations is an important aspect of the drilling process in the oil and gas industry. The samples provide geological and geophysical data, enabling a reservoir model to be established. Core samples are typically retrieved using coring equipment, which is transported to a laboratory where tests can be conducted on the core sample. The coring equipment typically includes a core barrel provided with a drill bit on the lower end thereof. In use, the core barrel and drill bit are rotated such that the drill bit cuts into the formation and the sample to be retrieved enters into the inner bore of the core barrel within which it will be entrapped and brought to the surface of the well, at which point where it can be taken to a laboratory to be analyzed.
However, a major problem when coring is that the core sample can become jammed or can collapse in the barrel and so instead of obtaining for example a 30 meter core within a 30 meter core barrel, only a few meters of core may be obtained within the inner bore of the core barrel if it jams and accordingly that 30 meter potential core sample is lost forever.
In recent years there have been some attempts to monitor the entry of a core into the barrel and one recent prior art system for doing so is disclosed in International PCT Patent Publication No. WO2006/058377 and which uses a core sample marker (32) (or “rabbit” as such equipment is known in the industry) located inside the inner core barrel 16 (see FIG. 4). As the core enters the inner barrel (16), the core pushes the rabbit (32) upwards and such upward movement is observed by using longitudinally spaced apart length markers (36, 38) and a location sensor (34). Accordingly, the distance traveled by the rabbit (32) can be transmitted in a signal to a signal receiver at the surface of the well. However, although there is some disclosure of providing a pressure sensor, a temperature sensor and possibly a rotational sensor, the information that can be sent to the operator at the surface is substantially limited to monitoring the entry of the core sample into the inner barrel and therefore it is not possible to foresee if a jam is likely to occur with the prior art system shown in PCT Publication No. WO2006/058377. Furthermore, the core barrel apparatus shown in International PCT Publication No. WO2006/058377 suffers from the disadvantage that the rabbit (32) will inherently to some extent inhibit the entry of the core sample into the inner core barrel.